The refinement of gamma-chymotrypsin has continued by real-space electron density fitting using both automated computer procedures and computer graphics. Additional refinement using differential fourier synthesis combined with model building using soft constraints is underway. Comparison with other known serine proteases is planned. A 2.5 A map of the acid protease from Rhizopus Chinensis has been calculated. The backbone of the entire molecule can be traced. Although the sequence data remain incomplete, a majority of amino acid assignments can be deduced with confidence based on sequence homology with other acid proteases and the quality of the map. A molecular model is under construction by means of computer graphics. Refinement methods developed for use with gamma-chymotrypsin should be useful in improving the model using 2.0 A data that are already available. BIBLIOGRAPHIC REFERENCE: Subramanian, E., Swan, I.D.A., Liu, M., Davies, D.R., Jenkins, J.A., Tickle, I.J. and Blundell, T.L.: Homology among acid proteases: Comparison of crystal structures at 3 A resolution of acid proteases from Rhizopus chinensis and Endothia parasitica (x-ray diffraction/inhibitor binding/enzyme cleft). Proc. Natl. Acad. Sci. U.S.A. 74: 556-559, 1977.